Chip CMOS optical element

ABSTRACT

CMOS optical receiver and optical transmitters are described. The optical receiver is formed from a CMOS CCD which is modified to immediately output all information indicative of incoming light, i.e., with no transfer gate. The optical transmitter is formed of a modulation window device. Both the optical transmitter and optical receiver are located on-chip with a microprocessor and form the I/O for the microprocessor. Since the modified I/O is serial, a serial to parallel converter, and parallel to serial converter are provided.

This is a Divisional of Application Ser. No. 09/248,350, filed Feb. 08,1999, now U.S. Pat. No. 6,147,366.

BACKGROUND

Optical technology allows high speed wireless communication. Opticalsignals can be carried at very high bandwith and low noise. Such signalscan also travel for much longer distances, e.g. on the order ofkilometers. Many optical receivers are used to receive signals that havebeen sent over tens of kilometers. Since the signal is received inoptical form, there are also fewer problems with ground loops, crosstalk and noise.

Photoreceptors for optical signals are often formed in non-standardsubstrates such as InP or GaAs to obtain the high sensitivity that isnecessary for receiving optical signals that have traveled over the longdistances. These substrates are often incompatible with the CMOSsubstrates used for many other IC processes, e.g. microprocessors.

Optical transmitters similarly have been formed in non-standardsubstrate types and materials. In addition, optical transmitters oftenrequire substantial power to be generated on the optical chip, e.g. togenerate the light for transmission.

Some optical transmitter configurations form a semiconductor laser onthe chip. However, these systems are also relatively difficult toreliably manufacture.

The high bandwidth capability of optical signals allows the medium tocarry much information.

Modern processors require high data input and output. Typicalconnections to a processor are made in parallel to provide the requiredthroughput. This requires large numbers of connection pins. For example,a 128-bit processor may use 128 input lines for the data input. Asprocessors become more powerful, it becomes increasingly difficult toprovide enough pins to allow the desired connections.

The high bandwidth of optical technology could allow serialcommunication at much higher speed over fewer lines. This could reducethe pin count. However, the non-standard semiconductor technology hasmade this an unattractive option.

Modern chip fabrication is often done in CMOS. If a different kind offabrication technology is used for the optical sensor or transmitter,that optical sensor or transmitter is preferably formed on a totallyseparate substrate from the CMOS sensor.

The power consumption of the optical transmitters makes it even moreunattractive, since processors already have extreme power consumptionand cooling requirements. Adding additional power consumption isundesirable.

SUMMARY OF THE INVENTION

The present system describes CMOS compatible optical devices on chipwith CMOS circuits, preferably CMOS microprocessors.

The circuit is formed with an silicon substrate with a CMOS circuitusing a charge coupled device formed in the substrate, using a logicfamily that is compatible with CMOS. The optical receiver has an inputconnected to receive a serial stream of optical information. The outputof the optical receiver is connected to the circuit. This opticalreceiver hence forms the input for the circuit.

In an alternate system, an optical transmitter is formed in thesubstrate, from a logic family that is compatible with CMOS. The opticaltransmitter has an input that is connected to receive output from themicroprocessor. It has an output that modulates a light source, from anexternal source, according to the output from the microprocessor. Thisoptical receiver hence forms the output for the microprocessor.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects will now be described with respect to theaccompanying drawings, in which:

FIG. 1 shows a diagram of a CMOS microprocessor with an on-chip opticalreceiver formed from a special CCD device;

FIG. 2 shows a diagram of a CMOS processor on chip with an opticaltransmitter;

FIG. 3 shows a CMOS processor with both optical transmitter and opticalreceiver, both on chip; and

FIG. 4 shows a multiple processor system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present disclosure describes techniques of using CMOS andCMOS-compatible fabrication techniques to form an optical device, e.g.,an optical transmitter and/or an optical receiver, on a substrate alongwith associated CMOS circuitry, e.g., a processor, which can be amicroprocessor such as a digital signal processor or DSP. The processorreceives and transmits information using the on-chip optical devices.For example, the optical receiver is formed on the same substrate withthe microprocessor. The incoming data to the processor, e.g., from thehard disk, BIOS and I/O is input to the microprocessor via the on chipoptical receiver.

The optical transmitter is also preferably formed on the same substratewith the processor to carry the processor output.

In a preferred mode, all microprocessor I/O is via single channel serialcommunication, using a single, high speed line for each of input andoutput.

A first embodiment is shown in FIG. 1. A single silicon substrate 100 isshown with a processor portion 102 that is formed with CMOS circuitry. Alight sensor portion 103 is formed using a formation process compatiblewith CMOS, e.g., CMOS, NMOS, PMOS, or general MOS, or any featurecompatible with any of PMOS, NMOS, or CMOS. The electrical output of theCMOS light sensor is coupled to the processor 102. In operation, theCMOS light sensor produces a signal indicative of incoming light. Thesignal indicative of incoming light is an electrical signal which iscoupled to the processor 102. Processor 102 accepts its input via theoptical signal, and takes some action based on the electrical signalapplied thereto.

The present system is preferably used in communication in personalcomputing. Such communication typically sends light for much shorterdistances than the distances which are typical for communication.Communication devices, for example, may require the light to travel tensof kilometers. In personal computing, a few inches to 100 meters is amore typical value. Since the light travels smaller distances,less-sensitive optical receivers can be used.

According to the present system, a charge-coupled device or “CCD” isused. The CCD is inherently light sensitive. Charge coupled devices havetypically been used as a register for charge. The charge is acquired bya photosensitive device which converts incoming light photons to charge.Typical such photosensitive devices include a photodiode or a photogate.The charge is stored by the CCD.

A typical use of a CCD is shown in U.S. Pat. No. 4,479,139. An incomingphoton is converted to charge by a photodiode and stored in the CCD.CCD's are often made using CMOS processes, as described for example inU.S. Pat. No. 4,642,877.

A special CCD is used as a short haul communications photoreceptor. TheCCD is placed on the substrate with CMOS circuitry, e.g. a processor,that carries out some action based on the content of the incoming lightsignal. This configuration has a number of advantages. CCD technology isalready well-developed for applications such as video capture. CCDs arealso inherently low noise devices. In addition, CCDs inherently operateserially which is the preferred mode for this operation.

In the preferred mode, the optical device is aligned to the underlyingboard 90 using a ball grid array technique. These techniques are wellknown in the art. A number of balls with solder are provided. Whenheated, the solder melts, and its surface tension forces the componentinto proper registration.

Hence, ball grid array technology can be used to align the opticalinterfaces between chip and board.

The detailed structure of the CCD light receiver is shown in FIG. 1.Incoming light 99 is received either as laser light, or some othercollimated light, or from a fiber. The incoming light 99 is coupled to amicrolens 101 formed on the top surface of silicon substrate 100. Theincoming light is coupled to the light sensor portion 103 which is shownincluding a silicon nitride layer and a silica layer. The CMOS CCD isphotosensitive, and hence produces an output signal on the line 105 thatis proportional to the amount of light 99 that is received. The outputcurrent 105 is first buffered by a source follower 114, and output tothe processor 102 within the silicon substrate 104.

A conventional CCD might include a buried charge storage area. Typicallya thin P++ type region 110 and a N+ region 112 below the P++ region formthe charge storage area along with perhaps additional doped regions,e.g., N− region 114. The charge is conventionally not allowed to leavethe storage area freely, i.e., it is integrated in the charge storagearea. A readout gate is located between the charge storage area and acharge transfer area, also called a shift register portion. The shiftregister portion is typically shielded by metal to prevent that areafrom accumulating charge from stray incoming light.

This embodiment is a special CCD that is different from the standard CCDwhich integrates the charge for an integration time. This system, incontrast, outputs instantaneous readout of the incoming light. The lightcauses P++ layer 110 and N+ layer 112 to produce charge. A currentindicative of the charge is output from the N+ layer to the electricalline 105, as produced. A source follower 114 in the CMOS circuitryreceives the instantaneous value. When the charge is sufficient toforward bias the gate junction of the source follower 114, thetransistor turns on, to signify a change in state. Conversely, when thelight is not present, the charge dissipates, and the source follower 114turns off. All this is instantaneously applied to the source follower114 without a separate transfer gate.

Of course, the opposite sense to that described herein is also possible,e.g. by using a PMOS source follower.

The high speed serial information as received by the source follower isconverted to parallel by an on-chip serial to parallel converter, andused to provide data to the parallel input connections of themicroprocessor.

Additional substrate layers, including N− layer 114 and additional P andN layers may be provided to enhance the charge production. Any of thewell-known standard techniques for CCD can be used with the aboveteaching, to form a CCD which immediately outputs the light-indicativesignal without integrating.

A second embodiment refers to a optical transmitter formed in a siliconsubstrate along with CMOS circuitry that is associated with the opticaltransmitter. This embodiment uses an optical switch as a short-haulcommunication photo-transmitter. The optical switch is formed of siliconusing a process compatible with CMOS. For example, this can use anoptically movable mirror. The silicon window forms a modulation window200 in an optical waveguide 202. The modulation window can beselectively turned on and off under control of an electrical signal inthe bond wire 204 connecting the silicon CMOS chip 210 to the opticalportion 199.

The modulation window can be formed in a number of different ways,including, but not limited to, a movable mirror of the type described inU.S. Pat. No. 4,938,555. Light from an external light source 220 isused. The output light 222 is coupled into the waveguide 199 and drivento the modulation window 220. The light is selectively allowed to passunder control of the electrical signal on the wire 204 which iscorrespondingly driven from a CMOS element 207 in silicon chip 210. Inthe case of a digital optical transmitter, the light is allowed to passwhen the CMOS switch 207 is on, and light is prevented from passing whenthe CMOS switch 207 is off. The power and amount of light is dependenton the external light source 220 which is separate from the substrate209 on which the processor 210 and optical portion is formed. Moreover,since the light source is off the chip, its power consumption isexternal to the chip. This avoids the necessity to consume power on thechip. This allows the processor to run cooler and hence be more linear.Therefore, rather than generating light, this photo-transmittermodulates light through the waveguide.

The microprocessor produces parallel output information, which isconverted to serial by an on-chip parallel to serial converter 206. Theoutput of the P/S converter drives the CMOS switch.

A specific preferred embodiment is shown in FIG. 3. The chip of FIG. 3includes a CMOS light sensor 300 of the type described in FIG. 1 toreceive input to processor 302. A CMOS light valve 304 of the type shownin FIG. 2 is also on the same substrate 299 to output the informationfrom the processor.

FIG. 4 shows a specific preferred embodiment using two processors 400,402 on separate substrates 410, 420. The two processors each have anassociated optical transmitter and receiver of the type described above.The first substrate has a transmitter 405 and receiver 408. The twodevices hence communicate optically.

Although only a few embodiments have been described in detail above,those of skill in the art recognize that many modifications are intendedand predictable from the disclosed embodiments. For example, other CMOScompatible optical devices can be used, including an active pixelsensor, or a digital mirror. Other circuits besides the microprocessorcan be driven.

All such modifications are intended to be encompassed within thefollowing claims.

1. A CMOS circuit, comprising: a silicon substrate having aCMOS-fabricated circuit therein; and an image sensor device, formed onsaid silicon substrate using an MOS formation process, and configured ina way to produce a substantially instantaneous readout of an amount ofincoming light incoming thereto without integrating said light andproducing an electrical signal indicative thereof, said electricalsignal forming an input signal for said CMOS fabricated circuit.
 2. Acircuit as in claim 1, wherein said CMOS-fabricated circuit is amicroprocessor.
 3. A circuit as in claim 2, wherein said microprocessoroperate on input information in parallel form, and further comprising aserial to parallel converter, converting serial information from saidincoming light to parallel information for said microprocessor.
 4. ACMOS circuit, comprising: a silicon substrate having a CMOS-fabricatedcircuit therein; and an optical modulating device, formed on saidsilicon substrate using an MOS formation process and configured in a wayto produce a light blocking action when an output of saidCMOS-fabricated circuit is a first logic state, and to produce a lightpassing action when said output of said CMOS-fabricated circuit is asecond logic state, said optical modulating device having a first endthat receives light from an external light source.
 5. A circuit as inclaim 4, wherein said CMOS-fabricated circuit is a microprocessor.